As chat markup is very similar to the markup used for the questions, I think there should be a preview WMD editor beside the input textfield in the chat. It would be useful; the markup is used less rarely in chat than in questions, but it's still used quite a lot, and I regularly find messages not displaying how I wanted. After the message appears wrongly, I only have a rather limited amount of time to edit it. I would like if I had a good preview before I posting it.
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I don't think it's necessary at all. Chat messages are generally much shorter than your average question or answer post, where you spend more time formatting and the preview is necessary before you post something that might not be legible due to formatting errors.
Think of chat messages as being similar to comments; you don't see a preview of those either. Mostly because the majority of chat messages and comments don't need/have formatting.